Page 19 - Issue 3 2023

TECHNICAL

would have resulted in a TC factor
close to unity for all three bushings –
valid for bushings H1 and H2 but not
for H3.
• Bushing H3 would have been
removed in this scenario due to a Table 3: Pre-installation C1 test results for new H3 replacement bushing, 10 kV LF PF and 250 V NB DFR
>1% PF value. However, had the
bushing been tested at 10°C instead
of at about 30°C, its measured and
temperature-corrected PF test results
by look-up tables may have been low
enough to pass acceptance criteria as
a normal, service-aged unit.

Given the LF PF test results, the utility
performed NB DFR tests on the high-side
bushings. 1 Hz and 505 Hz measurements Table 4: 10 kV overall transformer LF PF test results after H3 bushing replacement
from this testing are provided in Table
2. These tabular results affirm the good With the problem corrected in the overall winding insulation test results, the utility
condition and rating of bushings H1 and staff then complemented their now-routine procedure with a 10 kV LF PF test on the
H2 but escalate the assessment of the H3 installed H3 replacement bushing (Table 5).
bushing to an investigate rating. The 10 kV measured LF PF test result for the H3 replacement bushing was acceptable.
However, the ITC temperature-corrected LF PF result is about 1,5 times the measured
NB DFR test results for bushing H3 value, while the 1 Hz measured, and ITC temperature-corrected results are nearly the
(the blue trace in Figure 1) revealed
the following two worrisome electrical
characteristics and the utility replaced
the bushing.
• A bushing in good condition will have
an ITC temperature-corrected PF value
≤1% at 1 Hz. The ITC temperature-
corrected 1 Hz PF test result of
bushing H3 was 7,92 % (Table 2).
• The 10 kV LF PF test result, 1,1 %
(Table 2), and the LF PF test result
at 250 V, a typical NB DFR test
voltage, taken from Figure 1, 0,8 %,
are different. This implies a voltage
dependence of the LF PF test results.
Figure 1: NB DFR measured test results for the high-side 69 kV bushings
Before installing in the transformer, LF
PF and NB DFR testing was performed on
the H3 replacement bushing to confirm
its integrity. LF PF and NB DFR test
results indicated the bushing’s good (G)
condition (Table 3) (Figure 2).
After replacing the H3 bushing, the
utility repeated the overall LF PF tests
on the transformer to observe what
effect the H3 bushing replacement had
on the overall HV winding insulation test
characteristics (Table 4). There was a
notable improvement. Figure 2: Pre-installation NB DFR test results for H3 replacement bushing

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